1. Field of the Invention
This invention relates generally to the use of low-level fuel gas emissions from natural and man-made sources and more particularly to an apparatus and method for the capture and use of low-level fuel gas emissions to produce electricity or other useful work.
2. Description of Field and Related Art
The following references are relevant to the present application:
Acheson, et al. U.S. Pat. No. 4,202,169, describes a gas turbine system for the recovery of power from fuel gases having a low heating value, i.e., below about 80 Btu/scf, and usually in the range of 35 to 70 Btu/scf, has an external catalytic combustor. The catalytic combustor is divided into a primary and a secondary catalytic combustion chamber with a heat exchanger between the two combustion chambers. In the preheater the low heating value gas mixed with combustion air is passed in indirect heat exchange with products of combustion from the first combustion chamber before the low heating value gas is delivered to the first combustion chamber. The turbine system is particularly advantageous in recovering power from low heating value gas in which the combustibles are hydrocarbons, primarily methane.
Ricks, U.S. Pat. No. 4,209,303, describes a method and apparatus is disclosed for recovery of combustible gas formed from combustible refuse or vegetable matter in an enclosed space from which the combustible gas is collected. Water can be injected into the apparatus and the decay process initiated and promoted by activating a heating element projecting upwardly from the base of the apparatus into the material undergoing decomposition. The combustible gas contains a substantial proportion of methane.
Zison, U.S. Pat. No. 4,442,901, describes a method of collecting landfill gas from a landfill comprising providing a porous collector in the landfill having a relatively broad collection zone in the path of migrating landfill gas, controlling the pressure in the collector to induce the landfill gas near the collector to flow into the collector, removing the landfill gas from the collector, and substantially excluding air from the atmosphere from entering the collector when the collector is collecting landfill gas.
Zison , et al. U.S. Pat. No. 4,469,176, describes landfill gas recovery system, the breakthrough danger is minimized, and the system efficiency is improved, by providing pressure-equalizing low-impedance gas paths such as aggregate-filled. symmetry trenches positioned within the landfill and surrounding, at least partially, the primary collection zone. The symmetry trenches may be connected to the system""s low-pressure source to serve as secondary collectors. A sensing trench positioned within the landfill along the periphery of the collector""s zone of influence can be used to monitor the collector pressure and to automatically maintain it at a safe level. The sensing trench can also serve as a secondary equalizing path in heterogeneous landfills. Hot spots may advantageously be tapped by auxiliary collectors whose pressure level bears a predetermined proportional relationship to the primary collector pressure.
Moilliet, U.S. Pat. No. 4,493,770, describes a method which heat can be recovered by biological generation of heat upon aeration of refuse, such as garbage or sludge, in an aeration vessel by introducing oxygen-containing gas, such as air, in a closed cycle to thereby enrich gas withdrawn from the vessel with the oxygen, typically by reintroduction of gas withdrawn from an upper gas portion of the vessel, after introduction of additional oxygen, for example controlled by a valve into a lower portion of the contents of the aeration chamber. Control can be effected automatically, by a control unit through a valve or manually; automatic control can be effected, for example, by sensing oxygen or carbon dioxide concentration by suitable sensors within the vessel. To permit recovery of methane of high quality in a subsequent decomposition and methane recovery container, material withdrawn from the aeration vessel is degassed in degassing chambers for example by storage for about xc2xd hour, and venting of emanating gases. Control of valves regulating flow from, and to, the aerating vessel and the degassing chambers permits preheating of freshly introduced refuse by the material withdrawn from the degassing chambers in a counter flow heat exchanger, while preventing possible escape of non-aerated substances from the vessel by isolating the aeration vessel during introduction of new refuse, and emptying only a chamber of said degassing chamber system.
O""Brien et al., U.S. Pat. No. 4,681,612, describes a recycle process for the separation of landfill gas containing a wide variety of impurities into a carbon dioxide product stream and a fuel-grade-pressurized methane product stream, the process providing for the removal of both the impurities and the carbon dioxide in a cryogenic column as a bottom stream, the separation of the methane from the overhead product stream by a membrane process, and, optionally, the removal of impurities from the carbon dioxide bottom stream in a separate purification column, to recover a high-quality, liquid, carbon dioxide stream.
Nobilet et al., U.S. Pat. No. 4,769,149, describes a process for recovery of energy from waste and residues is disclosed. The residues, after sieving, are subjected to bacterial digestion in a methanization reactor and the solid phase of the digestate is then subjected to incineration in a furnace supplying a heat recuperator, the furnace being supplied with complementary combustible by the methane coming from the digester, while the circuit of the fumes downstream of the recuperator is used for heating by at least one secondary circuit, the magma in the course of treatment in the digester and/or the sludge separated from the digestate before recycling thereof towards the digester.
Watson et al., U.S. Pat. No. 5,059,405, describes a process and apparatus for removing the impurities from a gas stream produced from a landfill such that essentially pure carbon dioxide and methane is recovered. After the landfill gas is mechanically dewatered, the gas is filtered of particulate solids and aerosols and purified by removing sulfur compounds using zinc oxide columns, removing halogens using activated alumina columns, removing hydrocarbons using activated charcoal columns, and oxidizing remaining impurities using potassium permanganate impregnated activated alumina columns. Lastly the gas is incinerated in a boiler/incinerator combustion furnace to produce an exit stream containing essentially pure carbon dioxide and air, which is further treated in a conventional carbon dioxide treatment process.
Siwajek, U.S. Pat. No. 5,842,357, describes a process for concentrating and recovering methane and carbon dioxide from landfill gas includes absorption of commonly occurring pollutants using a reduced amount of carbon dioxide absorbent which itself may be an in situ derived and recoverable constituent. Separated methane may be concentrated into a high heating value fuel, and a highly pure food-grade carbon dioxide product may also be recovered. Process streams may be used to provide fuel for compression and refrigeration and/or to regenerate carbon dioxide absorbent.
In addition to the above prior art references we know that methane emissions, from many sources, such as cattle, landfills, marshes, swamps and from coal, natural gas and petroleum exploration and production seeps into the atmosphere in very low concentrations. The U.S. EPA estimates the aggregate amount of human-related sources of such methane emissions are 70 percent of the total and are over 30 million tons of methane emitted annually in the U.S., causing global warming and other environmental problems. About 19% of the methane emitted due to human activities comes directly from farm animals, 20 percent from oil and natural gas operations, 36% from landfills, 10 percent from coal mines, 9 percent from animal manure and the rest from other sources. Methane emissions from natural sources such as wetlands and marshes constitutes 30 percent of the methane, perhaps another 13 million tons annually in the U.S. Worldwide emissions of methane are at least ten times as large.
Methane is a very potent greenhouse gas, with 25 times the potential for global warming compared to carbon dioxide. Methane is produced naturally, is non-toxic, and its emissions are not regulated. However, the United States Environmental Protection Agency (EPA) has become concerned about the potent global warming effects of methane, and now has a strong Methane Energy Branch.
Enormous quantities of methane are emitted from natural and man-made sources each year. Methane is evolved when organic matter breaks down in an atmosphere starved of oxygen. Landfills emit methane in low concentrations. Marshes and swamps also produce and emit methane, once known as xe2x80x9cmarsh gasxe2x80x9d. Methane is also generated during the production of several crops, such as rice, where the field is fully or partly submerged in water during plant growth. The digestive process of farm cattle converts about roughly ten percent of their food intake into methane, emitted directly from the animal itself. For example, a ruminant adult cow""s digestive system produces over 100 kg or about 220 pounds of methane a year, approximately 5,000 cubic feet. Similar natural processes convert the organic matter in swamps and water treatment facilities to methane. Much of this methane is emitted in very low concentrations, and simply becomes a part of the atmosphere, and a major contributor to global warming. Methane is also emitted from natural gas, coal and petroleum exploration and recovery operations. Each of these sources of methane has been well documented by the EPA. The EPA states, xe2x80x9cAs a contributor to climate change, methane is second only to carbon dioxide. Over the last two centuries, methane concentrations in the atmosphere have more than doubled, largely due to human-related activities. Reductions of about 10 percent in emissions from these anthropogenic sources would halt the annual rise in methane concentrations, effectively contributing to mitigation of climate change.xe2x80x9d The EPA estimates that over 30 million tons of such methane are emitted in the U.S. alone.
Methane today is used in a highly concentrated form. Natural gas is about 90% methane. The xe2x80x9cbestxe2x80x9d landfills produce about 50% methane; that is, the gas as evolved at ground level is only one-half methane. It is increasingly recovered for the production of electricity. However, most methane is emitted in far lower concentrations and it is simply not considered practical to recover it for useful benefit. Except in certain cases, such as landfills, where the recovery of high-concentration gases is sometimes feasible, there exists no practical method for recovery of most freely evolved methane today, and, because it is slightly lighter than air, it rises into the atmosphere to exacerbate global warming. Methane recovery from crop sources is not a known practice today.
The prior art teaches the use of methane in the generation of electricity by collecting high concentrations of this gas, but does not teach an apparatus and method for recovery and use of very low concentrations of methane from natural and man-made sources. The present invention fulfills these needs and provides further related advantages as described in the following summary.
The present invention teaches certain benefits in construction and use, which give rise to the objectives described below.
This invention provides a process and apparatus for collecting and using extremely low concentrations of methane for producing electricity and heat. The methane that is vented into the air is collected and concentrated by covering the emissions area, trapping the methane and local air. Methane continues to seep into the container and is allowed to concentrate over time. When the concentrations are high enough to usefully recover the energy (about 1% by volume or less), the. gas is drawn into a catalytically combusted turbine. No other fuel is needed. Methane concentrations in the top of the container are naturally higher because methane is lighter than air. The higher concentrations are used to increase power output. Precautions must be taken for safety and concentrations should be maintained sufficiently low to ensure safety. This method may be used for any other low-concentration gases that may be oxidized to produce heat.
The apparatus and method described here provides a means to collect and use such gas for the production of electricity and useful heat in a turbine in a manner harmonious with the environment. The apparatus consists of a means for collecting such gas and allowing the gas concentrations to increase over time; a means for further local concentration of the methane by virtue of its specific gravity being significantly lower than air; a means for delivering the methane-laden air mixture to a turbine; a means for compressing and heating the mixture; a means for catalytic combustion of the mixture in a relatively simple catalytic combustor; a means for controlling the power output; a means for protecting against premature combustion and its attendant hazards. While methane gas is used for illustration in this narrative, any other gas or combination of gases or vapors that are able to be exothermically oxidized may be used with this system with suitable modifications.
As stated, a building structure encloses a gaseous mixture of air and a combustible fuel. Air is obtained from the atmosphere, and the gaseous fuel is obtained from natural evolution and diffusion processes associated with decomposition of materials, as from landfills, and gaseous digestion products from livestock, crop growth processes, etc. A process control system is engaged for drawing off the gaseous mixture, at a selected air-fuel ratio, from the structure. The selected gaseous mixture is drawn from the building, through a compressor and then a pre-heater, into a catalytic combustor where the mixture is burned and directed into a turbine for producing work. This work is preferably converted into electricity by a generator driven by the turbine. A process controller senses process variables such as temperature, pressure, flow rate, fuel concentration, etc. so as to assure that combustion cannot occur prematurely, but does occur most efficiently in the catalytic combustor. Process heat is used for preheating the mixture to be burned.
A primary objective of the present invention is to provide an apparatus and method of use of such apparatus that provides advantages not taught by the prior art.
Another objective is to provide such an invention capable of using low level gaseous fuels currently regarded as not practically usable.
A further objective is to provide such an invention capable of controlling the efficient combustion of such fuels to produce useful work.
A still further objective is to provide such an invention capable of safe and cost effective operation.
Other features and advantages of the present invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.